Power supply units, in particular switched-mode power supplies for connection to an AC mains, in the simplest case have a rectifier downstream of which an electrolytic capacitor is connected. The charging current profile of the capacitor leads to severe harmonics on the mains and loads said mains with a high apparent power. The mass use of power supply units within the last twenty years has for these reasons already led to serious problems in the power supply grids.
A brief presentation of these problems and various solution proposals, which are intended to lead to a balanced mains load, can be found in the article "Comparison of Standards and Power Supply Design Options for Limiting Harmonic Distortion in Power Systems", by Key and Lai, in "IEEE Transactions on Industry Applications" Vol. 20, No. 4, July/August 1993, pages 688 to 695. One of the circuits which are illustrated there and are also used in practice has a so-called "boost converter". A virtually sinusoidal input current profile can even be achieved using said boost converter, by means of a complex controller. As is noted in the article, however, the relatively high costs and relatively poor reliability, which is governed by additional components, preclude widespread introduction to the market.